Immunity and viral serology . Patient guide
HIV (4th generation antigen + antibody) Blood Test
Also known as: HIV combo test, HIV Ag/Ab test, HIV p24 antigen + antibody
What is HIV
The HIV 4th generation test detects both the p24 antigen (a viral protein measurable about 2 weeks after exposure) and HIV-1/2 antibodies (measurable about 3 to 4 weeks after exposure) in one assay. It is the UK standard for serological HIV screening. Most infections are detectable from 4 weeks and BHIVA/BASHH 2020 guidance regards a negative 4th generation test at 45 days (6 weeks) as conclusive in the absence of further exposure.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in qualitative (reactive or non-reactive). Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Non-reactive (negative) | No HIV antigen or antibody detected | No HIV infection at the point of testing (window period applies) |
| Reactive (initial screen) | Antigen or antibody detected | Always confirmed with a second assay before any diagnosis is made |
| Window period (combo test) | conclusive at 45 days (6 weeks) post-exposure | BHIVA/BASHH 2020: a negative 4th generation test at 45 days does not require a 12-week repeat |
What it is
HIV is a chronic viral infection that targets CD4 T-lymphocytes. The 4th generation combination test is a single laboratory assay that looks for two things in one blood sample: p24, a viral capsid protein produced in large amounts during acute infection, and IgG/IgM antibodies that the immune system makes against HIV-1 or HIV-2 from about 3 to 4 weeks onwards. Combining the two shortens the diagnostic window compared to antibody-only testing.
Why a clinician would order it
HIV serology is offered as part of comprehensive sexual health screening, after a potential exposure (with the understanding that the 4th generation test is regarded as conclusive at 45 days, around 6 weeks, after the exposure per BHIVA/BASHH 2020), in any new relationship for peace of mind, in pre-employment or insurance health checks, in pregnancy planning, and as part of pre-transplant BBV screening. HIV-positive patients can be treated safely on effective antiretroviral therapy with a normal life expectancy when treated early.
If your level is outside the range
Symptoms of low HIV
- A non-reactive result is reassuring at the point of testing. After a recent exposure the test must be repeated at 45 days (6 weeks) to clear the window period before HIV can be confidently excluded per BHIVA/BASHH 2020.
What low can indicate. No HIV infection at the time of testing. Important context: if the exposure was less than 45 days before testing, early infection may not yet be detectable, and a repeat 4th generation test at 45 days (6 weeks) is recommended per BHIVA/BASHH 2020.
Symptoms of high HIV
- Acute HIV (2 to 6 weeks after exposure) can cause fever, sore throat, rash, swollen lymph nodes and tiredness (a "seroconversion illness"), though many people are asymptomatic. Untreated chronic HIV eventually presents with opportunistic infection, persistent diarrhoea, weight loss and certain cancers.
What high can indicate. A reactive screen always triggers a confirmatory test before any clinical decision. A confirmed positive HIV result means HIV-1 or HIV-2 infection, now treated as a chronic, manageable condition with highly effective antiretroviral therapy. Linkage to a specialist HIV clinic is arranged with the patient.
Testing tips
No fasting required. Time testing at 45 days (6 weeks) after any specific exposure for a conclusive result; an earlier 4-week test will detect most infections but is not regarded as conclusive on its own per BHIVA/BASHH 2020. Confidential and ID-protected results are standard in private practice. Post-exposure prophylaxis (PEP) is time-critical and is started within 72 hours via a sexual health clinic or A&E, not via a blood test.
Where you can get this tested
HIV (4th generation antigen + antibody) is included in the following WMG Health panels. Same-day appointments at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Symptoms often investigated with HIV (4th generation antigen + antibody)
HIV (4th generation antigen + antibody) is commonly tested when patients present with the following symptoms. If any of these resonate with you, the linked guides explain what to look for and which test pathway is appropriate.
Related markers
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.
Common questions about HIV
What is a normal HIV range?
Non-reactive (negative): No HIV antigen or antibody detected (No HIV infection at the point of testing (window period applies)). Reactive (initial screen): Antigen or antibody detected (Always confirmed with a second assay before any diagnosis is made). Window period (combo test): conclusive at 45 days (6 weeks) post-exposure (BHIVA/BASHH 2020: a negative 4th generation test at 45 days does not require a 12-week repeat). Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low HIV result mean?
No HIV infection at the time of testing. Important context: if the exposure was less than 45 days before testing, early infection may not yet be detectable, and a repeat 4th generation test at 45 days (6 weeks) is recommended per BHIVA/BASHH 2020.
What does a high HIV result mean?
A reactive screen always triggers a confirmatory test before any clinical decision. A confirmed positive HIV result means HIV-1 or HIV-2 infection, now treated as a chronic, manageable condition with highly effective antiretroviral therapy. Linkage to a specialist HIV clinic is arranged with the patient.
Do I need to fast or prepare for the HIV blood test?
No fasting required. Time testing at 45 days (6 weeks) after any specific exposure for a conclusive result; an earlier 4-week test will detect most infections but is not regarded as conclusive on its own per BHIVA/BASHH 2020. Confidential and ID-protected results are standard in private practice. Post-exposure prophylaxis (PEP) is time-critical and is started within 72 hours via a sexual health clinic or A&E, not via a blood test.
Can I order a HIV blood test privately in London?
Yes. WMG Health offers HIV as part of bespoke panels and several pre-built panels at our 134 Harley Street clinic. Results are clinician-reviewed by a GMC-registered doctor within 4 hours for the most common assays. All panels are custom-built around your specific question; bookings via /contact/ or 020 3239 3378.